Bacterial culture medium

ABSTRACT

A bacterial culture medium used in testing for the susceptibility of bacteria to anti-folate, anti-bacterial agents is substantially improved for this use by the incorporation therein of purified thymidine phosphorylase of bacterial origin.

This invention relates to culture media for microbes and in particularto culture media used in testing for the susceptibility of bacteria toanti-folate anti-microbial agents such as sulphamethoxazole (SMX) and/ortrimethoprim (TMP).

It has been known for a number of years that culture media in common useare often unsuitable for determining sensitivity of bacteria tosulphonamides or trimethoprim, i.e. agents interfering with thesynthesis of folates in these organisms. This unsuitability manifestsitself by giving long tailing end-points when the serial dilution methodis used and by partial growth within the inhibition zones when thediffusion method is employed. It has been shown by Bushby (Med. J. Aust.Special Supplement (1973) 1: 10) and Koch and Burchall (AppliedMicrobiology (1971) 22: 812) that thymidine is a very potent reversingagent of the inhibiting activities of sulphonamides and trimethoprim.

In 1945 Harper and Cawston (J. Path. Bact., 57: 59) showed that whenlysed horse blood was added to a poor susceptibility test medium, itcould convert this into a satisfactory one. Since this early work, andthat of several other workers, it has become common practice to includelysed horse blood in antibacterial susceptibility test media, in orderto reduce the partial growth often observed within the inhibition zonesproduced by sulphonamides. More recently this method has also been shownto be similarly effective in testing with respect to trimethoprim(Bushby, Postgraduate Med. J. (1969) 45: 10; and Darrell et al., J.Clin. Path, (1968) 21: 202).

Harper and Cawston established that (a) lysed horse blood was moreeffective than whole blood in neutralizing sulphonamide-antagonizingsubstance(s), (b) blood of several other species was inactive, (c) theactivity of the lysate increased with incubation time and temperature(up to 30°) and (d) the lysate did not effect the reversal ofsulphonamide-inhibition by p-aminobenzoic acid. They concluded that thelysed blood contained a factor which neutralizessulphonamide-antagonizing substance(s).

This so-called Harper-Cawston Factor is effective only with media whichcontain a moderate level of thymidine, i.e. from about 0.1 to 15 μg/ml.Below 0.1 μg/ml the activity of the drugs is not antagonized; thus,removal of such a small amount of thymidine has no effect on the druginhibition observed. At very high levels of thymidine, i.e. greater than15 μg/ml, the activity of Harper-Cawston Factor is not sufficient toovercome the reversal of the activities of the sulphonamides andtrimethoprim, possibly because the high concentration of thymine,produced as a result of the cleavage of thymidine, can replace the muchmore active thymidine in the reversal.

The Harper-Cawston Factor has been reported to be thymidinephosphorylase (Bushby in Trimethoprim/Sulfamethoxazole in BacterialInfections: A Wellcome Foundation Symposium. Ed Bernstein & Salter,Churchill Livingston, Edinburgh & London, 1973, p. 31-38; and Bushby,Med. J. Aust. Special Supplement, 1973, 1: 10-18) It has been pointedout in the latter reference that "although thymidine interferes with thein vitro activity of TMP/SMX, it is not usually present in animals insufficiently high concentrations to affect the in vivo activity."

One disadvantage of including lysed horse blood in a culture medium isthat it imparts a reddish brown color to the medium; the greater theamount of horse blood, the deeper the color. This coloration isundesirable since it greatly interferes with the assessment of bacterialgrowth after incubation. For example, in the case of fluid media theincreased color decreases the transparency of the media making opticaldensity measurements far less accurate. with solid media there is adecrease in contrast of the agar making it more difficult to preciselymeasure inhibition zone sizes and to determine the presence or absenceof partial growth within the zones of inhibition. Furthermore therequirement of the addition of lysed horse blood to bacterial culturemedia means that the media are virtually impossible to define, asomewhat undesirable characteristic. A further disadvantage of usinghorse blood is that it is commercially available in very limited supplyand from ony a very few suppliers world-wide.

It has recently been found that the addition of the isolated andpurified enzyme thymidine phorphorylase of bacterial origin, to a widevariety of commonly used growth media, improves those media forsusceptibility testing of bacteria to anti-folate drugs.

According to one aspect of the invention there is provided a compositionfor testing the susceptibility of bacteria to anti-folate drugs whichcomprises a bacterial growth medium in combination with purifiedthymidine phosphorylase of bacterial origin.

Addition of the enzyme thymidine phosphorylase improves many media forsusceptibility testing of bacteria to anti-folate drugs, such assulphamethoxazole and trimethoprim, for example Mueller-Hinton Broth andAgar, Oxoid Sensitivity Test Broth and Agar, Wellcotest Sensitivity TestAgar, Brain Heart Infusion Broth and Agar, and Typtone Soya Agar.

Thymidine phosphorylase has previously been purified from bacteria suchas Salmonella typhimurium, Bacillus cereus, Bacillus stearothermophilus& Haemophilus influenzae and particularly from a strain of Escherichiacoli requiring thymine and methionine for growth. This latterpurification involved an extremely lengthy process consisting ofprecipitation, fractionation, several chromotographic steps and dialysis(Schwartz, M., 1971, Eur. J. Biochem. 21: 191-198). The enzymepreparation recovered from this process, however, was only 25-fold purerthan the crude cell extract.

It has recently been found that certain strains of bacteria, inparticular a certain strain of E. coli, produce inordinate amounts ofthymidine phosphorylase under appropriate growth conditions and thatthis enzyme may be isolated and purified by applying the cell extract tospecific adsorbents and eluting it therefrom to give a much higher yieldof a much purer preparation than has heretofore been achieved. Moreover,this new isolation/purification procedure is adpatable to large scaleproduction of the enzyme.

According to a second aspect of the invention there is provided aprocess for preparing purified thymidine phosphorylase of bacterialorigin, which process comprises the steps of extracting the crude enzymefrom bacteria into an aqueous medium, and subjecting the extract to afractionation procedure which includes adsorption, chromatographic anddialysis steps. This process is characterized in that (1) a strain ofbacteria is selected which has the capability of producing highconcentrations of thymidine phosphorylase under appropriate growthconditions and (2) the crude cell extract obtained therefrom is appliedto a calcium phosphate gel absorbent containing substantially equivalentamounts of Ca⁺⁺ and PO₄ ≡, preferably as a first step in purification.Preferably, the eluant is subsequently contacted with DEAE celluloseand/or cellulose-epichlorohydrin triethanolamine (ECTEOLA-cellulose),with dialysis against water or a suitable buffer being carried out afterelution from DEAE-cellulose and before adsorption of ECTEOLA-cellulose.

Escherichia coli B-96 (ATCC 13473) is eminently advantageous for thepurposes of the present invention. Salmonella typhimurium LT-2 (ATCC15277) is an example of another strain of bacteria which produces largeamounts of thymidine phosphorylase and thus is useful in the practice ofthis invention. In certain cases, where a more thermostable enzyme isdesirable, the enzyme isolated from B. stearothermophilus has proveneffective.

The E. coli strain ATCC 13473 may be cultured in a minimal salts mediumcontaining a suitable carbon source and additional purines.Alternatively, the bacteria may be cultured in a yeast extract medium. Acrude extract of the enzyme may then be made by sonication of thebacterial cells in phosphate buffer followed by centrifugation to removethe cell debris.

The crude extract is for instance admixed with a small amount of calciumphosphate gel and then centrifuged to remove unwanted protein. Thesupernatent so obtained may then be admixed with a further aliquot ofcalcium phosphate gel and the enzyme absorbed thereto. The enzymeactivity may be eluted from the gel by sequential washings withphosphate buffer. The enzyme may be adsorbed of DEAE-cellulose, washedand eluted therefrom. After dialysis, the preparation may be adsorbed toECTEOLA-cellulose and eluted therefrom.

Monitoring of the elutions for enzyme activity, at all stages of thepurification, may conveniently be carried out using a spectrophotometricassay at a selected wavelength.

The enzyme so purified can be conveniently made available as asuspension in aqueous ammonium sulphate.

The enzyme is also present in a number of vertebrate tissues and can bepurified therefrom, but the levels in mammalian tissues are generallymuch lower than in bacteria. Furthermore purified microbial thymidinephosphorylase is many times more active than the purified mammaliancounterpart.

Indeed horse blood has been found to contain about 40 to 100 units ofthymidine phosphorylase activity (as defined herein) per ml. The E. colisonicate of Example 1 contains greater than 1000 times thisconcentration. Thus, the advantages of an economical method of preparingthe purified enzyme from a bacterial source are many and significant.Not only is there a more readily available, inexpensive source of theenzyme, but the process for extracting and purifying it is much moreeconomical.

The concentration of thymidine phosphorylase incorporated into the mediais preferably in the range of about 2 to 200 units of enzyme activity/mlof medium, more preferably between 5 and 100 units/ml and mostpreferably between 7 and 50 units/ml. One unit of enzyme activity ofpurified enzyme is that amount of the enzyme which catalyzes theformation of one nanonole of thymine/minute from a one millimolarsolution of thymidine at 25° C in the presence of 200mM potassiumphosphate buffer at pH 7.4.

Media, to which purified thymidine phosphorylase has been added, may besuitable for testing the susceptibility to anti-folate drugs of avariety of organisms, such as Streptococcus pyogenes, Staphylococcusaureus, Vibrio comma, Erysipelothrix rhusiopathiae, Serratia marcescens,Klebsiella pneumoniae, Kleb. aerogenes, Sal. typhosa, E. coli, Shigellaflexneri, Shig. dysenteriae, Enterobacter aerogenes, Entero. cloacae,Citrobacter freundii, Proteus vulgaris, Pr. mirabiles, Pr. rettgeri, andPseudomonas aeruginosa. Strep. faecalis is an outstanding exception,because with this organism, thymine is as effective as is thymidine invirtually reversing the activity of inhibitors of folate reductase, eg.trimethoprim.

The purified enzyme may be added to the desired medium at any suitablestage of manufacture or preparation. For example, it may be addedaseptically as a sterile solution after autoclaving of the medium, andwhen the temperature has dropped to about 50°-55° C. After the enzymehas been added, the medium should be processed as soon as possible sothat the enzyme-treated medium is not maintained at 50°-55° C. for morethan about 5-10 minutes in order to minimise inactivation of the enzyme.

According to the present invention, in a third aspect there is provideda method of preparing a composition suitable for testing thesusceptibility of bacteria to antifolate drugs which comprises theadmixture of a purified preparation of thymidine phosphorylase ofbacterial origin with a bacterial growth medium.

One particular advantage of the composition so produced is that is islight colored and transparent, which facilitates the accurate evaluationof bacterial growth in the determination of bacterial sensitivity toantifolate drugs.

According to yet another aspect of the invention there is provided astabilized thymidine phosphorylase preparation containing ammoniumsulfate. The formulation of increased stability may be suspension of theenzyme in an ammonium sulfate solution.

It is known in the prior art that thymidine phosphorylase of bacterialorigin is stable at -20° C. but that at 4° C. activity decreases at asignificant rate. It has now been found that formulations of thepurified enzyme can be made remarkably stable to decomposition by addingammonium sulfate to the preparation provided that the protein content ofthe preparation is at least 5mg protein/ml. The protein content need notnecessarily all consist of the enzyme. Concentrated solutions of theenzyme (5mg protein/ml or greater) in phosphate buffer containing 10%ammonium sulfate, for example, may be stored for long periods of timewith little or no loss of activity. Stable suspensions of thymidinephosphorylase in aqueous ammonium sulfate may also be prepared.

The following examples illustrate the invention but do not limit it inany way.

EXAMPLE 1 Thymidine Phosphorylase Purification

E. Coli B-96 (ATCC 13473) was grown in aerated vessels at 34° C in aminimal salts medium containing Na₂ HPO₄ (18.9 g/1), KH₂ PO₄ (6.3 g/1),MgSO₄. 7H₂ O (0.2 g/1), (NH₄)₂ SO₄ (2.0 g/1), adenosine (0.5 g/1), andcasamino acids (8.0 g/1). The cells were harvested by centrifugationwhen the optical density of the culture at 600 nm (without dilution)reached 2.0. The following operations were carried out at 3° C unlessotherwise specified. The cell paste (25 g) was suspended in two timesits weight of 5 mM potassium phosphate buffer, pH 8.0 (Buffer A). Thecell suspension was sonicated in 5 ml aliquots, each for two 12 secondperiods with a 50 second cooling interval. A Branson Model 5125 SonicOscillator was used at a power setting of six. The sonicates were pooledand centrifuged for 20 minutes at 48,000 × g. To the supernatant (StageI - see Table I below) was slowly added 25 ml of a calcium phosphate gelsuspension (31 mg dry solid per ml, aged at 3° C for 5 months). Theresulting suspension was stirred for 10 minutes and then centrifuged at12,000 × g for 5 minutes. The supernatant was mixed with an additional100 ml of the calcium phosphate gel suspension, and the mixture wasstirred for 10 minutes and centrifuged at 9,700 × g for 15 minutes. Theresulting gel pellet was washed with Buffer A (100 ml) by resuspensionand centrifugation at 9,700 × g for 15 minutes.

The enzyme activity was eluted from the gel pellet by two sequentialwashings; the first with 10 mM potassium phosphate buffer, pH 8.0 (100ml) and the second with 20 mM potassium phosphate buffer, pH 8.0 (100ml). The two washes were combined (Stage II).

The remainder of the procedure was carried out at 25° C. The combinedwashes were applied to DEAE-cellulose column, 1.8 cm in diameter by 6 cmhigh, which was previously equilibrated with 20 mM potassium phosphate,pH 6.4 (Buffer B). The loaded column was washed with Buffer B (100 ml),and the enzyme was then eluted with a linear gradient of phosphatebuffer. This gradient was prepared by adding with thorough mixing 200 mMpotassium phosphate buffer, pH 6.4 (200 ml) to a mixing chamber, whichwas originally filled with Buffer B (200 ml), at such a rate as tomaintain constant volume within the mixing chamber. The fractionscontaining the highest enzyme activity were pooled (Stage III) anddialyzed using a cellulose dialyzer tubing (1/4 inch diameter) againstwater (2 1) for 1 hour. The dialysis was repeated, and then thedialysate was applied to an ECTEOLA-cellulose column (1.8 × 5 cm)previously equilibrated with Buffer A. The loaded column was washed withBuffer A (100 ml). The enzyme was then eluted with a linear gradienteluant prepared as above using a mixing reservoir initially filled withBuffer A (200 ml) into which 200 mM potassium phosphate buffer, pH 8.0(200 ml) was siphoned by gravity as the column solution proceeded. Thefractions containing enzyme activity were pooled (Stage IV) andsufficient ammonium sulfate added to give a suspension of the enzyme in80% ammonium sulfate solution. This overall procedure resulted in a100-fold increase in specific activity with respect to protein and thevirtually complete removal of nucleic acids.

Thymidine phosphorylase activity was assayed spectrophotometrically bymeasuring the decrease in absorbance at 290 nm accompanying thephosphorolysis of thymidine to thymine. At 290 nm and pH 7.4 thymidinehas a higher extinction coefficient than thymine (Δε=-480M⁻¹ cm⁻¹) Theassay mixture contained 200 mM potassium phosphate buffer, pH 7.4 and 1mM thymidine in a total volume of 2.5 ml. One unit of enzyme activity isthat amount which catalyzes the formation of one nanomole of thymine perminute from thymidine at 25° C.

Table I summarizes the results of the purification described in thisexample. This procedure has been scaled up 60-fold with similar results.

                                      TABLE I:                                    __________________________________________________________________________    Purification of Thymidine Phosphorylase from                                  Escherichia coli B-96                                                                              Activity          Activity  Purifi-                                  Stage                                                                             Vol. Units Total  Protein                                                                            Units Yield                                                                             cation                       Procedure   No. (ml) ml    Units  mg/ml                                                                              mg prot.                                                                            %                                fold                                                                          __________________________________________________________________________    Sonicate Supernatant                                                                       I  65.5 106,000                                                                             6,943,000                                                                            40    2,650                                                                              100 --                           Eluate from Calcium                                                           Phosphate Gel                                                                              II 206  27,600                                                                              4,685,600                                                                            0.99  27,880                                                                             82  11                           DEAE-cellulose                                                                Eluant      III 52   80,500                                                                              4,186,000                                                                            0.61 132,000                                                                             60  50                           ECTEOLA-cellulose                                                             Eluant      IV  66   50,700                                                                              3,346,200                                                                            0.19 266,800                                                                             48  101                          __________________________________________________________________________

EXAMPLE 2 Solid Medium

Mueller-Hinton (Difco) Agar was prepared in the normal manner andautoclaved. After the medium had been removed from the autoclave andallowed to cool to 50°-55° C, a sterile solution of purified thymidinephosphorylase as prepared in Example 1 containing sufficient enzyme togive a final concentration of 40 units of the purified enzyme per ml ofmedium, was added aseptically. The medium was thoroughly mixed, pouredinto sterile Petri dishes and allowed to cool to room temperature.

EXAMPLE 3 Sensitivity Testing

Mueller-Hinton Agar (Wilson) and Brain-Heart Infusion Agar (Difco) wereprepared in the normal manner and autoclaved (three batches of each).After removal from the autoclave, one batch of each medium was pouredinto sterile Petri dishes. The other two batches of each medium wereallowed to cool to 55° C. To one batch of each was then added sufficientlysed horse blood to give a final concentration of 5%, and to the finalbatch of each medium was added a sterile solution of thymidinephosphorylase sufficient to give a final concentration of 40 units/ml.Each batch was thoroughly mixed and then poured into sterile Petridishes. After cooling to room temperature, each agar plate was seededwith 2 ml of a diluted (10⁻⁴) overnight broth culture (Mueller-Hintonbroth) of E. coli. Excess fluid was removed, and the plates were allowedto dry. Filter paper discs containing 1.25 mg TMP, 1.25 mg TMP = 23.75mg SMX, and 23.75 mg SMX were then placed on the surface of the plates,which were then incubated at 37° C for 16 hours giving a lawn of notquite confluent growth. The zones of inhibition were determined andexpressed as the distance from the edge of the disc to the edge of thegrowth of uninhibited colonies.

                  TABLE II:                                                       ______________________________________                                        Sensitivity Testing                                                                           Size of Zone in mm                                                                                  TMP                                                                           2.36 1.25 μg                                   Reversing   TMP      SMX    + SMX                                   Medium    Agent       1.25 μg                                                                             23.75 μg                                                                          23.75 μg                             ______________________________________                                                  Thymidine                                                                     Phosphorylase                                                                             22(25)   17(20) 26(31)                                            40 Units/ml                                                         Brain-Heart                                                                             Lysed                                                               Infusion Agar                                                                           Horse Blood 14(22)   12(20) 21(29)                                  (Difco)   5%                                                                            Nil         (22)     (23)   (31)                                              Thymidine                                                                     Phosphorylase                                                                             24(27)   24(26) 32(36)                                            40 Units/ml                                                         Mueller-Hinton                                                                          Lysed                                                               Agar      Horse Blood 25(28)   24(26) 31(35)                                  (Wilson)  5%                                                                            Nil         (25)     (26)   (35)                                    ______________________________________                                         () Includes zone of partial inhibition.                                  

Mueller-Hinton agar is improved for sensitivity testing to about thesame degree by the addition of bacterial thymidine phosphorylase as bythe addition of lysed horse blood. However, the enzyme improvesBrain-Heart Infusion agar significantly more effectively than does lysedhorse blood. In addition, the virtually colorless plates producedaccording to this invention were read and evaluated much more easily andquickly than the colored horse blood plates.

We claim:
 1. A composition which is usable for testing thesusceptibility of bacteria to anti-folate agents comprising a bacterialgrowth medium in combination with purified thymidine phosphorylaseenzyme of bacterial origin wherein the concentration of thymidinephosphorylase enzyme in the growth medium is from 2 to 200 units ofenzyme activity/ml of medium.
 2. A composition as claimed in claim 1wherein the concentration is from 5 to 100 units/ml.
 3. A composition asclaimed in claim 1 wherein the concentration is from 7 to 50 units/ml.4. A composition as claimed in claim 1 wherein the thymidinephosphorylase is obtained from an Escherichia coli strain.
 5. Acomposition as claimed in claim 4 wherein the strain of E. coli is B-96(ATCC 13473).
 6. A composition as claimed in claim 1 wherein thebacterial growth medium is Mueller-Hinton Broth, Mueller-Hinton Agar,Oxoid Sensitivity Test Broth, Oxoid Sensitivity Test Agar, WellcotestSensitivity Test Agar, Brain Heart Infusion Broth, Brain Heart InfusionAgar or Tryptone Soya Agar.
 7. A method of preparing a compositioncomprising a bacterial growth medium and thymidine a bacterial growthmedium and thymidine phosphorylase of bacterial origin wherein theconcentration of thymidine phosphorylase enzyme in the growth medium isfrom 2 to 200 units of enzyme activity/ml of growth medium whichcomprises mixing a bacterial growth medium with a sufficient amount ofthymidine phosphorylase enzyme of bacterial origin/ml of bacterialgrowth medium which is usable for testing the susceptibility of bacteriato antifolate agents to produce 2 to 200 units of enzyme activity/ml ofbacterial growth medium.